The invention relates to so-called ball valves wherein a valve member which rotates to control fluid flow through the valve is characterized by a spherical surface which has sealed engagement to an annular seat for the closed condition of the valve.
The conventional valve member of a ball valve is a full sphere, except for a radial stem and a diametrically extending bore that is transverse to the stem direction. The valve-member or ball is actuable by limited rotation, e.g., 90.degree., about a valve-body axis of stem support, wherein said axis extends through the center of the sphere, intersecting and normal to the axis of the diametrically extending bore.
In the open condition of the valve, the diametrically extending bore aligns with cylindrical inlet and outlet ports or passages in the valve body, and an annular seal such as an elastomeric O-ring retained by one or both of these ports or passages is in peripherally continuous seated and sealing contact with the ball, encircling the adjacent end of the diametrically extending bore of the valve member. As the valve-member is actuated in the valve-closing direction, the valve-member bore and the inlet/outlet passage become progressively misaligned while the ball rotates with continuing seat engagement, thus reducing the sectional area available for inlet-to-outlet flow. When fully rotated to the valve-closed condition, a smooth spherical surface of the ball is circumferentially sealed to its seat, in total blockage of inlet-to-outlet flow.
The actuating operation of a conventional ball valve is thus characterized by the frictional resistance of the ball-to-seat engagement. For many applications, this friction can be reduced by appropriate choice of seat material and by careful attention to ball sphericity and to the accuracy of ball-stem mounting and rotation. But for other applications, as for controlled flow of cryogenic materials such as liquified oxygen, liquified hydrogen, liquified nitrogen, or other gases whether or not in liquid state, seating materials and engagements become sources of friction, wear, and leakage, to the extent that mechanical hysteresis is an on-going operational factor, and repair and maintenance expenses are relatively great.
U.S. Pat. Nos. 5,228,645 and 5,333,833 disclose ball-valve constructions wherein much of the indicated hysteresis is avoided by utilizing eccentric gyration of the rotary axis of the ball, wherein, for an initially closed condition of the valve, and in its initial phase of actuation, the throw of the eccentric is connected to retract the ball from its seat sufficiently for the ball to clear the seat, prior to a second phase of actuation involving ball rotation to open condition. To achieve this result, rotary drive of the ball has a lost-motion relation to direct drive of the eccentric. In its return stroke from full-open condition, the ball is directly driven in rotation back to a stop at which it has been oriented for its closed condition, and the final phase of eccentric rotation is operative to displace the ball center and thus the ball into contact and ensuing completely sealed engagement with the seat.
Although valves of said patents represent an important advance over prior ball-valve constructions, the fact remains that ball rotation is about an eccentrically gyrated axis, however small the eccentric throw; therefore, the final phase of valve closure is necessarily accompanied by a component of eccentric displacement, in the course of establishing sealed closure of the valve. Such constructions therefore provide a valve action which is short of a more desirable poppet action in the closing/sealing phase of valve operation.